The present invention relates to a time-axis correcting circuit for reducing time-axis error components included in a reproduced signal outputted by a recorded-data reproducing device, thereby to improve the quality of the reproduced signal.
A reproduced signal outputted by a recorded-data reproducing device includes time-axis error components because, for instance, the operation of the drive section is not always regular. Especially in a video signal reproducing device, such as a video disc player or a video tape recorder, the reproduced image may be poor in quality because of the time-axis error components included in the reproduced signal. That is, the time-axis error components greatly affect the reproduced image. Thus, it is essential to reduce such time-axis error components.
Time-axis correction of the reproduced signal may be achieved by a device such as a digital TBC (Time Base Correcting) circuit with which the reproduced signal is converted into a PCM (Pulse Code Modulation) signal. In this device, the time-axis error components are absorbed in a memory. However, the device is not suitable for consumer product use since it is large and expensive.
Examples of time-axis correcting circuits for consumer product use, which must be low in cost, are shown in FIGS. 1 and 2. In FIG. 1, reference numeral 1 designates a VDL (Variable Delay Circuit) for varying the phase between an input reproduced signal and an output signal in response to a control signal, 2 indicates a separating TSS (Timing Signal Separating) circuit for separating a timing signal component from the reproduced signal, 3 depicts a reference signal generating circuit (Ref. 0) for providing a reference signal having a frequency equal to that of the timing signal with no time-axis error component, 4 is a phase comparison circuit, and 5 a phase compensating circuit for determining the characteristic and stability of the closed-loop control system composed of the circuits 1, 2, 4 and 5. The closed-loop control system operates such that the timing signal separated from the output reproduced signal from the variable delay circuit 1 by the separating circuit 2 and the reference signal produced by the reference signal generating circuit 3 are subjected to phase comparison by the phase comparison circuit 4 to thus detect the time-axis error component of the timing signal. A time-axis error voltage, which represents the time-axis error component, is applied through the phase compensating circuit 5 as a control signal to the variable delay circuit 1.
In this arrangement, when a reproduced signal having a time-axis error component passes through the variable delay circuit 1, the signal is effectively shifted in time to thereby eliminate errors. However, in this time-axis error correcting circuit, it is difficult to make the frequency of the output reference signal of the reference signal generating circuit 3 exactly equal to that of the timing signal as in the case when no time-axis error component is present. Therefore, the DC component of the output time-axis error voltage of the phase comparison circuit 4 may exceed the dynamic range of the closed-loop control system, thus making the control system unstable.
In the time-axis correcting circuit shown in FIG. 2, the above-described difficulty has been eliminated. In FIG. 2, those components which have been previously described with reference to FIG. 1 are designated by the same reference numerals or characters, and a further description thereof will not be given. In FIG. 2, reference numeral 6 designates a flywheel oscillator for detecting only the time-axis error component from the input signal timing. The flywheel oscillator 6 is implemented with a PLL (Phase-Locked Loop) circuit. The closed-loop control characteristic of the PLL circuit is selected so that the phase of the output signal of the VCO (Voltage-Controlled Oscillator) circuit in the PLL is locked to a low-frequency time variation component of the received timing signal. The output of the flywheel oscillator 6 forms the output of the phase comparison circuit which detects the amount of phase shift of the timing signal with respect to the output of the VCO. The flywheel oscillator 6 is a stable closed-loop control device. Accordingly, the frequency of the reference signal is completely equal to the frequency of the timing data for the case where time-axis error is present. Accordingly, the drawback accompanying the time-axis correcting circuit of FIG. 1 is eliminated. That is, the time-axis correcting circuit of FIG. 2 operates stably.
On the other hand, in the circuit of FIG. 2, the time-axis error components in the reproduced signal have a distribution peculiar to the characteristics of the reproducing device in which a certain time-axis error component predominates. For instance, in the cases of a video disc player in which the disc turns at a speed of 1800 r.p.m., a time-axis error component at a frequency of 30 Hz predominates if the disc being played is eccentric. However, in the above-described conventional time-axis correcting circuit, the flywheel oscillator 6 operates to extract only the high-frequency time variations from the timing error signal. Accordingly, the second-described conventional time-axis correcting circuit suffers from a drawback in that it cannot completely suppress the dominant time-axis error components.
In view of the foregoing, an object of the present invention is to provide a time-axis correcting circuit having a simple arrangement and with which dominant and other time-axis error components are completely suppressed, thereby providing a high quality reproduced signal.